1. Field of the Invention
The present invention relates to an organic compound, a light-emitting element, a light-emitting device, an electronic device, and a lighting device.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL) (also referred to as EL element). In a basic structure of an EL element, a layer containing a light-emitting substance is provided between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting substance can be obtained.
An EL element is a self-luminous element and thus has advantages over a liquid crystal display element, such as high visibility of the pixels and no need of backlight, and is considered to be suitable as a flat panel display element. Another major advantage of such an EL element is that it can be manufactured to be thin and lightweight. Besides, the EL element has an advantage of quite fast response speed.
Since an EL element can be formed in a film form, planar light emission can be obtained; thus, a large-area element can be easily formed. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, the light-emitting element also has great potential as a planar light source applicable to a lighting device and the like.
EL elements can be broadly classified according to whether a light-emitting substance is an organic compound or an inorganic compound. In the case of an organic EL element in which a layer containing an organic compound used as a light-emitting substance is provided between a pair of electrodes, by applying a voltage to the light-emitting element, electrons from a cathode and holes from an anode are injected into the layer containing the organic compound and thus a current flows. The injected electrons and holes then lead the organic compound to its excited state, so that light emission is obtained from the excited organic compound.
The excited state formed by an organic compound can be a singlet excited state or a triplet excited state. Light emission from the singlet excited state (S*) is called fluorescence, and emission from the triplet excited state (T*) is called phosphorescence.
In improving element characteristics of the light-emitting element, there are many problems which depend on substances used for the light-emitting element. Therefore, improvement in an element structure, development of a substance, and the like have been carried out in order to solve the problems. For example, Patent Document 1 discloses a carbazole derivative having a high hole-transport property as a material that can be used for forming a light-emitting element with high emission efficiency.